1. Field of the Invention
This invention pertains to the field of microelectromechanical systems (MEMS) actuators.
2. Description of the Related Art
Electrically controlled actuators receive an electrical signal input and provide a mechanical output. The mechanical output provides power (force times displacement, per unit time) that can be used to move objects. Large, electrically controlled actuators are common in mechanical systems to control valves, pumps, switches and otherwise move objects.
Recent innovations require control of very small components that are formed on semiconductor substrates by conventional semiconductor fabrication processes. Groups of such components are known as microelectromechanical systems (MEMS). MEMS borrow design elements from their larger, conventional-size, functional equivalents, but must be adapted to semiconductor fabrication techniques and the dynamic effects of miniature size. An often essential part of MEMS are actuators that provide physical movement or force to other MEMS components in order to operate, or initiate, the MEMS device.
In U.S. Pat. No. 5,808,384 a photolithographic process is used to fabricate a MEMS having an actuator that controls switches, relays, and valves. This actuator consists of a coil and magnetic core to move a member. However, this actuator is capable of only a very small range of motion and is thus limited to particular applications in which a relatively small range of motion is required.
Certain MEMS devices include relatively large planar objects that require a means to position the planar object for operation. In so-called xe2x80x9cbillboardxe2x80x9d applications, a planar object is formed flat against a supporting wafer and must be moved upright for use (i.e., oblique to the wafer)xe2x80x94similar to how a billboard is arranged relative to its supporting ground surface.
U.S. Pat. No. 5,867,297 discloses such a billboard application in relation to a prior art MEMS optical scanner in which a mirror is fabricated in the horizontal plane (i.e., parallel to the wafer upon which the mirror is formed) and then lifted into a substantially vertical arrangement. The 5,867,297 patent states that a comb drive may be used xe2x80x9cto facilitate this processxe2x80x9d without disclosing how a comb drive could do so.
Electrostatic comb drives use principals of electrical capacitance to provide power to move an object. Electrostatic comb drives are comprised of two comb-shaped portions (xe2x80x9ccombsxe2x80x9d) that are arranged so that a set of fingers of one comb is interdigitated with a set of fingers of the other comb. When a voltage is applied across the two combs, a capacitance is created between fingers of the respective combs that creates an attractive force urging the combs to move toward one another.
Comb drives may be linear, such that each comb includes parallel fingers projecting from a straight backbone, or rotary, wherein the fingers project radially from a curvilinear backbone. Inherent disadvantages of comb drives are its small range of motion and the forces generated are small.
In contrast to the motion produced by a comb drive, raising a billboard requires a relatively large range of motion and may require a relatively large force to start the movement or to push the billboard into a jig. Other MEMS devices may also require a large range of motion, including adjustable optical systems that have a large focal length.
Prior art devices that use comb drives to move an object through a large motion must compensate for the comb drive""s small range of motion. One method uses two comb drives arranged orthogonal to one another to rotate a round drive wheel of a ratchet mechanism whereby the comb drives provide sequential pulling forces on the drive wheel and a pawl acts on the drive to prevent reverse motion so that the drive wheel rotates in one direction only. The ratchet mechanism is coupled to the object, such as the billboard structure, to raise the object to its desired orientation. Problems with such mechanisms include the large number or moving parts, which creates multiple failure modes and increases fabrication complexity, and the large area on the substrate (a so-called xe2x80x9cfootprintxe2x80x9d) that is necessary for the parts. In contrast, ideal MEMS actuators are simple, compact, and easily fabricated.
The present invention provides a wafer-mounted microelectromechanical systems (MEMS) actuator that receives electrical input and provides a mechanical output having a relatively large range of motion.
A preferred embodiment of a MEMS actuator of the present invention includes a drive mechanism coupled to a pallet, and a drive member that is slideably coupled to a substrate wafer. Operation of the drive mechanism moves the pallet in-and-out of contact with the drive member so as to incrementally move the drive member. Movement of the drive member may be used to move other objects, such as raising a billboard or moving a lens.
In preferred embodiments, the drive mechanism is a comb drive, although other types of drive mechanisms are also suitable.
The drive member includes opposed rows of drive teeth located on inner margins of a channel formed in the drive member. The pallet has opposite edges that each include a row of pallet teeth, and the pallet is located in the channel, between the rows of drive teeth. The pallet and drive member are arranged so that when the drive mechanism moves the pallet, the pallet teeth move into, and out of, engagement with respective rows of drive teeth on the drive member. As explained below, when the pallet teeth move into engagement with the drive teeth, the drive member is moved incrementally. Repeated engagements of the pallet teeth with the drive teeth cause the drive member to move a desired distance. In preferred embodiments, the drive member is elongate and slideably coupled to the substrate and constrained to move along a desired drive axis.
The pallet teeth and the drive teeth are shaped so that the pallet teeth can mesh with the drive teeth. A preferred tooth design is a sawtooth pattern wherein each tooth includes a leading side and a trailing side. As explained in greater detail below, the orientation and arrangement of the respective sides determine a direction of motion, motion increment size, force, and other properties.
The pallet and drive teeth may be shaped to accommodate a particular application. The pallet and drive teeth may be made symmetrical so that the drive member may be driven in two directions, e.g., forward and backward. The pallet and drive teeth may be shaped to provide a greater incremental motion upon each contact or the respective teeth may be shaped to provide less motion per increment, but more force. The shape of the teeth may be further arranged to provide other advantages as may be desired for a particular application.
In operation, the pallet oscillates between the legs of the drive member so that the pallet teeth push against the drive teeth. When the pallet teeth are aligned with the drive teeth, and the pallet is pushed against the drive member, the teeth simply mesh and the drive member does not move. However, when the pallet teeth are misaligned with the drive teeth, meaning that the respective teeth are not in meshing alignment, and the pallet is pushed against the drive member, the drive member moves, in response to the force from the pallet, until the teeth mesh.
Repeated oscillation of the drive mechanism is necessary to move the pallet into repeated engagement with the drive member so as to incrementally move the drive member through its full range of motion.
The several preferred embodiments of the present invention provide different means for the misalignment of pallet and drive teeth, referred to above. In a first embodiment, the opposing inner margins of the drive member have offset teeth and the pallet teeth on the two pallet margins are aligned with one another. Then, when the pallet contacts a first one of the two inner drive member margins, the pallet teeth and drive teeth are, or become, aligned. But, now the opposing pallet teeth are misaligned with the respective drive teeth of the second inner drive member margin. Then, when the pallet is moved into contact with the second inner margin the desired misalignment is provided so that the drive member must move to allow the pallet teeth and drive teeth of the second inner margin to mesh. When the pallet teeth and the second inner margin teeth are meshed, the respective pallet teeth and the first inner margin teeth are misaligned as desired. The process is repeated until the drive member has moved a desired distance.
In a second embodiment, the drive member drive teeth on the opposing margins are aligned, but the pallet teeth of the opposite margins are offset. The operation of this embodiment is similar to the first embodiment described immediately above. To wit, when a respective set of pallet teeth and drive teeth mesh, the other respective opposite set of pallet teeth and drive teeth are misaligned so that when the pallet is moved so as to urge that other respective set of pallet and drive teeth into contact, the drive member must move under the urging force of the pallet so that the teeth mesh.
In other embodiments, the desired misalignment may be provided by a ratchet and pawl mechanism, a biased dog, or other means that cause the pallet teeth and drive teeth to be misaligned just prior to urging the pallet teeth against the drive teeth.
A prior art method of fabrication of MEMS is a multi-user MEMS process (referred to as MUMPs). In general, the MUMPs process provides up to three-layers of conformal polysilicon that are etched to create a desired physical structure. The first layer, POLY 0, is coupled to a supporting nonconductive wafer. The second and third layers, POLY 1 and POLY 2, are mechanical layers that can be separated from their underlying structure by the use of sacrificial layers that separate the layers during fabrication and are removed near the end of the process. The POLY 1 and POLY 2 layers may also be fixed to the underlying structure (the wafer or lower POLY 0 or POLY 1 layer as the case may be) through openings, or vias, made by etching.
The MUMPs process also provides for a final top layer of 0.5 xcexcm thick metal for probing, bonding, electrical routing and reflective mirror surfaces.
Further information of the MUMPs process is available from Cronos Microsystems, Inc., 3021 Cornwallis Road, Research Triangle Park, N.C.
In preferred embodiments, the device of the present invention is fabricated by the MUMPs process. However, the MUMPs process may change as dictated by Cronos Microsystems, Inc., or other design considerations. The MUMPs fabrication process is not a part of the present invention and is only one of several processes that can be used to make the present invention.